1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor including a cap which covers a sensing element while being spaced by a predetermined distance from the sensing element.
2. Description of the Background Art
In recent days, an air bag system has been employed in most of automobiles on the market. Typically, an air bag system includes an acceleration sensor for detecting impact.
In order to make an acceleration sensor adaptable to incorporation in various types of automobiles, there has conventionally been efforts to reduce a size of an acceleration sensor, as well as its associated costs. For example, concerning a package for covering a semiconductor substrate which forms an acceleration detector and a signal processor of an acceleration sensor, metal which had been most widely used as a material for the package has been superseded by resin.
In the meantime, turning to a structure of an acceleration sensor, a sensing element including a mass body which is a movable portion for detecting an acceleration and the like is formed on a surface of a semiconductor substrate in an acceleration detector. Further, a glass cap is bonded to the surface of the semiconductor substrate in order to keep an admissible motion space of the mass body and prevent entry of dust, water or the like into the admissible motion space. By provision of the glass cap, the admissible motion space of the mass body is completely enclosed.
More specifically, a bonding frame which surrounds the sensing element formed on the semiconductor substrate in plan view is formed on the semiconductor substrate. It is noted that polysilicon doped with impurities which is used as a material for the sensing element is also used as a material for the bonding frame. The bonding frame is in contact with an end portion of the glass cap. With the bonding frame and the end portion of the glass cap being kept in contact with each other, the glass cap and the semiconductor substrate are bonded to each other by anodic bonding for purposes of improving heremeticity or the like (please refer to National Publication of Translation No. 2002-500961 and Japanese Patent Application Laid-Open No. 9-292409, for example).
However, since the bonding frame is formed of polysilicon doped with impurities, the above-described structure has suffered from problems associated with anodic bonding, as follows.
Upon application of a voltage in anodic bonding, the impurities contained in the bonding frame are precipitated out in a portion of the bonding frame near a bonding interface. As anodic bonding to bond the glass cap and the bonding frame is achieved by uniting together glass of the glass cap and polysilicon of the bonding frame, the precipitated impurities existing between the glass of the glass cap and the polysilicon of the bonding frame would reduce a bonding strength between the glass cap and the bonding frame.
That is, to allow the impurities to be precipitated out near the bonding interface would cause reduction of the bonding strength between the glass cap and the bonding frame.